Data communications using residential power lines are known in the art. An advantage of using the lines is that only peripheral infrastructure needs to be added to the existing power lines in order to transmit and receive the data communications. Amongst disadvantages of using power lines are the relatively considerable interference existing on the lines, such as voltage spikes and Gaussian white and colored noise, and the need to avoid introducing radio-frequency (RF) interference into the lines. Furthermore, power lines inherently attenuate RF signals heavily, since the lines are designed for efficient conveyance of low-frequency alternating current, or occasionally of direct current, and are not designed to support efficient transfer of higher frequency signals.
A number of different techniques are known in the art for mitigating the problems caused by communicating data over power lines. These techniques include error correction, differential and adaptive shift code keying (DCSK and ACSK), and direct sequence spread sequence transmission. Itran communications Ltd., of Beer Sheva, Israel, produce an ITM1 power line modem which is able to transmit data at 2.5 Mb/s. The ITM1 modem acts as a physical layer interface (PHY) between the power line and data transmission systems, and uses methods such as those above for overcoming the problems inherent in the power line.
Power line communication (PLC) systems known in the art use time-division and/or frequency-division and/or code-division multiplexing in order to maximize data transfer capacity. The multiplexing enables a number of users (persons and/or data terminals) within a relatively small range of distances to communicate, typically using an industry-standard packet data protocol, such as the Internet or an Ethernet protocol. While fixed time and frequency division multiplexing are relatively simple to implement, both lead to wasted bandwidth allocations, and lower throughput, since inevitably some of the fixed slots are under-utilized. Dynamic time and/or frequency multiplexing leads to more efficient use of bandwidth and thus higher throughput, closer but still less than the theoretical limit of 100% use of the available bandwidth, at a cost of more complex protocols for monitoring usage and allocating time slots or frequency bands. The multiplexing system is typically implemented at a media access control layer (MAC) level.
U.S. Pat. No. 5,929,750, to Brown, which is incorporated herein by reference, describes a power transmission network which transfers telecommunication signals. The network uses frequency conversion to convert relatively high-frequency data communications signals to lower frequency signals, of the order of 1 MHz, which are injected into and recovered from the power line system. Voice and data signals may be transmitted over all sections of the network by suitable detection, amplification and/or regeneration and reintroduction as and when necessary, and the network provides full duplex facilities so that signals may be transmitted and/or received in all directions simultaneously.
U.S. Pat. No. 6,101,214 to Hershey et al., which is incorporated herein by reference, describes a system for spread spectrum power line communication. The spread spectrum system uses a harmonic modulation transmitter and receiver for communicating over power lines.
U.S. Pat. No. 6,144,292, to Brown, which is incorporated herein by reference, describes communications apparatus generally similar to that described in U.S. Pat. No. 5,929,750 referenced above, including power lines having a plurality of phase conductors. The apparatus transmits data communications signals using a carrier frequency greater than 1 MHz, and uses frequency, time, and/or code division multiplexing techniques.
U.S. Pat. No. 6,194,996 to Okazaki et al., which is incorporated herein by reference, describes apparatus for transmitting data signals over power lines. Signal distortion is reduced by selecting appropriate components for the apparatus, which also uses spread spectrum techniques to transfer the data signals.
Because of the high power line attenuation, PLC systems known in the art are unable, of themselves, to transmit data over large distances, and are often limited to data transmission within one location. When a first PLC system needs to send data, such as an Internet packet, to a second PLC system at some distance from the first system, both PLC systems need to transfer the data to an intermediate system which is able to convey the data over the larger distance, such as via a telephone line. Furthermore, as described above, systems known in the art at best achieve bandwidths and corresponding throughputs close to but less than the theoretical maximum.